An artificial kidney apparatus is an extracorporeal blood circulation and processing system in which blood is removed from a patient, dialyzed to remove impurities and excess liquid, and returned to the patient.
In chronic care dialysis the patient typically is suffering from long term or end stage renal failure. Although the chronic care dialysis patient is quite ill, the patient's condition is usually relatively stable. The chronic patient typically visits a dialysis center periodically, approximately two or three times a week, to be dialyzed. Blood flows and amounts of fluids and impurities removed are relatively large and removed over a relatively short time. Acute care dialysis is more typical of temporary renal failure, such as from trauma. Under these circumstances the patient's body is often unable to withstand the relatively drastic and sudden changes that accompany chronic care dialysis. It is, therefore, desirable to dialyze the patient continuously at very low blood flow rates.
An objective common to virtually all extracorporeal circulation and processing systems for biological liquids, such as blood, and to infusion of biological liquids, such as blood or blood components, and pharmaceutical liquids, such as saline solution and intravenous medications, is to minimize or eliminate the infusion of undissolved air or gases (referred to simply as "air" hereafter) into the body of the patient. The air may take the form of fairly large bubbles or may be in the form of much smaller bubbles referred to as microbubbles. It is well known that the infusion of large bubbles of air can result in injury or death to the patient. The effects of a small number of microbubbles are less serious, but it is generally considered prudent to limit the total amount or rate of air infused in the form of microbubbles.
In chronic care dialysis air is typically removed and detected in a drip chamber located in a return line to the patient. The drip chamber is a relatively large volume reservoir with an air-blood interface configured to induce air bubbles to coalesce and leave the blood at the air-blood interface. Exemplary drip chambers are illustrated in U.S. Pat. No. 4,102,655 issued in 1978 to Jeffery et al., U.S. Pat. No. 4,666,598 issued in 1987 Heath et. al., and U.S. Pat. No. 4,681,606 issued in 1987 to Swan, Jr., et al. The possibility of infusing air into the patient is relatively easy to detect in a drip chamber because a build up of air will cause the level of blood in the chamber to drop. Necessary protective measures can be initiated when the blood level in the drip chamber drops below a predetermined level.
In acute care dialysis the blood flow rate through the artificial kidney apparatus is much lower than in typical chronic care dialysis. In acute care dialysis it is disadvantageous to use a drip chamber because of an increased possibility of blood clot formation due to low blood velocity and the presence of the air-blood interface. In order to detect bubbles at these low blood flow rates it is desirable to detect them directly in the tubing of the line that returns treated blood to the patient, rather than using a drip chamber. Of course, tubing type air bubble detectors may also be used advantageously in conjunction with chronic care dialysis artificial kidney apparatus and in any other situation where it is desired to detect the passage of air, such as in conjunction with the infusion of a pharmaceutical liquid or a biological liquid. Tubing type air bubble detectors may further be used advantageously in conjunction with blood apheresis and cardiovascular bypass procedures.
Furthermore, because acute care dialysis is continuous, the cumulative effects of microbubbles, while not well understood, may be more important in acute care dialysis than in chronic care dialysis.
Air detectors typically comprise a transmitting means and a receiving means. The transmitting means transmits a signal, such as an ultrasonic signal, through a chamber or tubing. The receiving means detects the signal and interprets it. The liquid and inclusions attenuate the signals differently. For example, blood and other liquids attenuate ultrasonic signals relatively little, while air and other gases attenuate ultrasonic signals relatively greatly. By monitoring the degree of attenuation of the ultrasonic signal as it passes through the chamber or tubing, it is possible to interpret a decrease in the strength of the signal received by the receiving means as indicative of the presence of a bubble. Ultrasonic air bubble detectors (UABDs) are described generally in U.S. Pat. No. 3,921,622 issued in 1975 to Cole, 3,974,681 issued in 1976 to Namery, U.S. Pat. No. 4,068,521 issued in 1978 to Consentino et al., U.S. Pat. No. 4,341,116 issued in 1982 to Bilstad et al., U.S. Pat. No. 4,418,565 issued in 1983 to St. John, U.S. Pat. No. 4,487,601 issued in 1984 to Lindemann, U.S. Pat. No. 4,607,520 issued in 1986 to Dam, U.S. Pat. No. 4,651,555 issued in 1987 to Dam and U.S. Pat. No. 5,191,795 issued in 1993 to Fellingham et al.
One major problem presented by using a UABD or other air detector on tubing is that the characteristics of the detection environment change over time during a given procedure and vary from procedure to procedure. For example, the ultrasonic signal output of the transmitting means may vary, the type of tubing may vary and its characteristics may change with time and the sensitivity of the receiving means may vary. In addition, the typical electronic components of a UABD have manufacturing tolerances which can cause variation between otherwise identical UABDs. These variations may not be significant in a chronic care dialysis procedure, because the relatively short duration of the procedure limits their effect. Furthermore, when detecting air in a drip chamber, variations in the detection environment have a less significant effect because a drip chamber air detector is essentially a blood level detector, requiring less sensitivity. The variability may be more important during acute care dialysis because the patient will be connected to the artificial kidney apparatus for an extended period of time when compared with chronic care dialysis, allowing for the accumulation of a greater total effect due to the variations. In some UABDs used in artificial kidney apparatus an automatic gain control (AGC) circuit adjusts the gain of an amplifier stage in the receiving means to maintain a relatively constant average output signal from the amplifier stage in order to compensate for variations in the detection environment.
U.S. Pat. No. 4,015,464 issued in 1977 to Miller et al. illustrates a UABD incorporating an AGC circuit that adjusts the gain of a receiving means amplifier simultaneously adjusting a transmitted signal level and a detected signal level to maintain the circuit in a marginally oscillatory state.
As stated above, the patient undergoing acute care dialysis will be connected to an artificial kidney apparatus system for extended periods of time. Because of this it is not only desirable to prevent infusion of bubbles above a certain size but it is also important that a volume of air infused in a given period of time in the form of bubbles smaller than the discrete bubble size limit, be limited.
In order for the ultrasonic signal to be transmitted through a tube properly the tubing holders used to hold the tube in position desirably positively retain the tube in a position where it is ultrasonically coupled to the tubing holders. Tubing holders such as that proposed in U.S. Pat. No. 4,418,565 issued in 1983 to St. John rely on friction and deformation of a resilient tube to hold the tube in place and do not positively retain the tube. Doors have been used to force the tubing into a tubing holder and to positively retain the tubing in place, but the doors can break off, creating a maintenance task.
It is against this background that the improved UABD of the present invention developed.